1. Field of the Invention
The present invention relates to a film forming method and a film forming apparatus, and specifically, to a film forming method and a film forming apparatus for spraying and depositing a raw material powder on a substrate or the like.
2. Description of a Related Art
Recent years, a technique of controlling crystal orientation in a solid material such as metal or ceramics in order to control and improve characteristics of the material has been attracting attention. For example, a functional material having an advantageous electric characteristic in a specific direction and a high-strength material strong in bending stress in a specific direction can be fabricated by orienting crystal grains. As a technique of fabricating such a functional material, an orientation control technique of performing treatment such as crystal growth, sol-gel deposition, HIP (hot isostatic pressing), or SIP (non-pressure sintering) in a strong magnetic field is known.
As a related art, Japanese Patent Application Publication JP-P2003-342100A discloses a method of controlling crystal orientation which method requires no pretreatment such as rapid quenching or rolling and is applicable not only to ferromagnetic material but also to non-magnetic metal, ceramics or organic material (page 1). In this method, crystal orientation of a material is controlled by reheating the material to solid-liquid coexistence temperature while applying a strong magnetic field thereto, or agitating the material in a solid-liquid coexistence temperature range in a coagulation process to separate crystal grains, and thereby, creating a condition in which individual crystal grains float in melt and freely rotate so as to rotate the crystal grains in a direction in which magnetization energy becomes the minimum.
Further, JP-P2004-131363A discloses a method of controlling crystal orientation to obtain a ceramics high-order structure having a single-layer or multi-layer, in which ceramics monocrystal particles are highly oriented, simply without requiring a mold or container for casting while enabling control of orientation and layer thickness of monocrystal particles (page 1). This method includes the steps of orienting individual particles in ceramics monocrystal particle suspension, which is formed by charging and dispersing particles in a solvent, by utilizing monocrystal magnetic anisotropy, and applying an electric field to the oriented suspension to deposit charged and oriented ceramic particles.
Furthermore, Uchikoshi et al., “Control of crystalline texture in polycrystalline alumina ceramics by electrophoretic deposition in a strong magnetic field” (J. Mater. Res., Vol. 19, No. 5, May 2004, pp. 1487-1491) discloses a principle of orientation control of a ceramics film by performing electrophoretic deposition in a strong magnetic field (page 1487).
However, according to JP-P2003-342100A or JP-P2004-131363A, since bulk material formation or film formation under high temperature environment or wet condition is mainstream, the steps are large in number, complicated, and troublesome. Further, when the fabricated functional material is applied to a device, a complicated step of forming a film pattern is further required. Accordingly, a method and an apparatus for more simply and easily manufacturing an orientation-controlled material are desired.
By the way, recent years, research on a film forming method called “aerosol deposition method” (hereinafter, also referred to as “AD method”) has been conducted. The AD method is a method of forming a thin film on the order of micrometers to a thick film on the order of millimeters by generating an aerosol by dispersing a raw material powder in a gas, injecting it from a nozzle and spraying it on a substrate or the like. Here, the aerosol refers to fine particles of solid or liquid floating in a gas. The AD method is also called “injection deposition method” or “gas deposition method”. In the AD method, film formation is performed in a mechanism that the raw material powder injected at a high speed collides against a lower layer, cuts into it and is crushed, and thus formed crushed surfaces adhere to the lower layer. This film formation mechanism is called “mechanochemical reaction”, and this reaction enables formation of dense and strong films containing no impurity, at normal temperature and in a dry condition. According to the AD method, since the nozzle is used when the aerosol is injected, a desired pattern can be formed by adjusting the aperture diameter, aperture shape, or the like of the nozzle. Accordingly, new material development utilizing such a special film formation mechanism and adoption in various applications are expected.
However, there is a problem that a film formed by the AD method (hereinafter, also referred to as “AD film”) falls short of property values such as piezoelectric performance of a bulk material having the same composition. Accordingly, development of a technique of improving property values of the AD film to surpass those of the bulk material or at least become equal to those of the bulk material is desired.